1. Field of the Invention
The present invention relates to a backlight unit and a Liquid-Crystal Display (LCD) device using the backlight unit. The invention is applicable to any other display device than the LCD device if it comprises sheet- or plate-shaped optical members arranged in a casing, for example, a display device employing an organic EL (ElectroLuminescence) element as a backlight.
2. Description of the Related Art
The LCD device has been extensively used as a monitor of the so-called Office Automation (OA) apparatus or television (TV) set because of its characters such as compactness, thinness, and low power-consumption. The LCD device comprises as its main components a LCD panel having a pair of opposing transparent substrates and a liquid crystal layer interposed between the substrates, and a backlight unit for generating backlight illuminating the panel. The backlight unit comprises as its main components a light source, optical members (e.g., a light-guiding plate, a diffusion sheet, a lens sheet, and a polarization sheet) for converting the light emitted from the source to uniform illumination light, and a casing for holding and/or covering the light source and the optical members.
The above-described optical members have their own assembling orders in the casing, which are defined in advance. Moreover, the attitude or layout of these members with respect to the casing, such as the front and back, the top and bottom, and the left and right of the respective members, is also defined beforehand. However, the appearance of these members is similar and therefore, these members are likely to be placed at a wrong order and/or in a wrong direction. As a result, there arise unfavorable conditions or states, such as degradation of luminance, and non-uniform distribution of luminance. To confirm the right assembling order and the right attitude of these optical members after the assembling operation of the backlight unit is completed, a technique as shown in FIGS. 1A to 1C was developed. This technique was disclosed in the Patent Document 1 (the Japanese Non-Examined Patent Publication No. 11-352476 published in 1999), in particular, on pages 2 to 4 and FIG. 1.
As shown in FIGS. 1A and 1B, ear-like parts 118a and 118b are respectively formed on a first lens sheet 117a and a second lens sheet 117b in their predetermined regions outside the angle of view in such a way that the parts 118a and 118b are not entirely overlapped on each other. On the other hand, an eliminated or depressed part 120 is formed on a frame-shaped casing 119 to receive the ear-like parts 118a and 118b of the sheets 117a and 117b. When the sheets 117a and 117b are assembled in the casing 119 in a predetermined order, the parts 118a and 118b are overlapped in the state as shown in FIG. 1C, where the part 118a is partially seen through the central gap or slit of the part 118b. Thus, the assembling order of the first and second lens sheets 117a and 117b can be discriminated by observing in a specific direction the parts 118a and 118b inserted into the depressed part 120 of the casing 119.
By using the above-described technique disclosed by the Patent Document 1, the count and inserted order of the lens sheets 117a and 117b can be visually inspected after the assembling operation of the backlight unit is completed. However, with this configuration, the depressed part 120 of the casing 119 has a size large enough for receiving both of the ear-like parts 118a and 118b and therefore, the sheets 117a and 117b may be arranged in an unfavorable order. As a result, whether or not the assembling order of the sheets 117a and 117b is right cannot be judged in the assembling stage of the backlight unit. If some problem or malfunction is visually found after the assembling operation of the backlight unit is completed, the unit needs to be reassembled.
Moreover, with the above-described configuration disclosed by the Patent Document 1, when the count of the lens sheets is limited to approximately two, whether the assembling order of the lens sheets is right can be judged by visually checking the state of the overlapped parts 118a and 118b. However, when the count of the lens sheets used is equal to three or more, the overlapping state of the parts 118a and 118b will be complicated. Therefore, it is not easy to judge the rightness of the assembling order of the lens sheets from the overlapping state of the parts 118a and 118b. In particular, optical members such as the lens sheets, the light-guiding plate, the diffusion sheet, and the polarization sheet are all translucent and thus, it will be more difficult to judge the rightness of the assembling order of the optical members.